It is a common desire in gas-turbine technology to increase the efficiency of combustion of the fuel-air mixture employed in such engines. It is also desirable to be able to obtain a good mix between fuel and air, which is not greatly dependent on the load, which is placed on the engine.
Various measures have been invented to enhance the fuel-air mixing process. One such is disclosed in U.S. Pat. No. 4,991,398, issued to assignee United Technologies Corporation. FIGS. 1(a) and 1(b) show a basic principle of a technique described in this patent. An annular combustor 10 has disposed at a dome end 12 thereof a number of fuel nozzles 14. The nozzles 14 are circumferentially spaced apart in two rows—a first, radially inner row 16 and a second, radially outer row 18, referred to a longitudinal axis 20 of the combustor. The nozzles of one row interleave with those of the other row, so as to create a triangular configuration shown as G in FIG. 1(a). Each of the nozzles has its own swirler device and the directions of the swirl in each case are shown by the arrows 24. Combustion air is introduced into the combustion chamber at an axially intermediate point 22.
As can be seen in FIG. 1(b), the fuel spray 26 (nominally in the form of a cone) emanating from the nozzles in row 18 interleaves with the fuel spray 28 emanating from the nozzles in row 16, the result being a mutual reinforcement of the two flows. Because of this reinforcement, the fuel tends to be distributed more uniformly throughout the combustion chamber, and fuel-air mixing is more intense. Indeed, intense combustion starts quite close to the dome within distance C1, increasing the axial extent of the intense burning to distance E1, before the combustion becomes diluted. This enables the combustor to be made shorter in length, thereby saving space and weight.
A development of the arrangement just described is set out in U.S. Pat. No. 6,360,525, issued to Alstom Gas Turbines Ltd. In this patent (see FIGS. 2(a) and 2(b)) an annular combustor 30 has a similar arrangement of nozzles to that in U.S. Pat. No. 4,991,398, but this time the dome end 32 of the combustor is made in two radially adjacent sections, namely sections 32a and 32b, which are arranged at an angle 34 to each other at a mid-point 36 of the combustor. The two rows of nozzles 38 are provided in respective sections 32a, 32b and, since these sections are inclined with respect to the normal to the longitudinal axis 36, the spray cones 40, 42 emanating from the nozzles are radially inclined toward each other in an interleaving manner. The longitudinal centrelines 41, 43 of the spray cones leaving the nozzles cross each other at an axial point 44, which forms a circle over the whole annular combustor.
This mutual inclination of the two flows radially toward each other creates a stronger mixing action between the flows than is the case in the FIG. 1 arrangement. This, in turn, enhances the uniformity of combustion in the primary combustion zone. In addition the distance C1 in FIG. 1(b) is reduced, enabling the combustor to be further shortened in the axial direction.